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The rapidly declining population of bright quasars at z ≳ 3 appears to make an increasingly smaller contribution to the ionising background at the H I Lyman limit. It is thererfore generally thought that massive stars in (pre-)Galactic systems may provide the additional ionising flux needed to complete H I reionisation by z ≳ 6. A galaxy-dominated background, however, may require that the escape fraction of Lyman continuum radiation from high-redshift galaxies is as high as 10%, which is somewhat at odds with (admittedly scarce) observational constraints. High escape fractions from dwarf galaxies have been advocated, or, alternatively, a so-far undetected (or barely detected) population of unobscured, high-redshift faint AGNs. Here we examine the latter hypothesis and show that such sources, to be consistent with the measured level of the unresolved X-ray background at z = 0, can provide a fraction of the H II filling factor no larger than 13% by z ≃ 6. The fraction rises to ≲27% in the somewhat extreme case of a constant comoving redshift evolution of the AGN emissivity. This still calls for a mean escape fraction of ionising photons from high-z galaxies of ≳10%. [ABSTRACT FROM AUTHOR]

A recent revision of black hole scaling relations indicate that the local mass density in black holes should be five times higher than previously determined values. The local black hole mass density is connected to the mean radiative effciency of accretion through the time integral of the Active Galactic Nuclei (AGN) volume density and a significant increase in the local black hole mass density would have interesting consequences on AGN accretion properties and demography. One possibility to explain a large black hole mass density is that most of the black hole growth is via radiatively ineffcient channels such as super Eddington accretion; however, this solution is not unique. Here we show how it is possible to accommodate a larger fraction of heavily buried, Compton-thick AGNs, without violating the limit imposed by the spectral energy density of the hard X-ray and mid-infrared backgrounds. [ABSTRACT FROM AUTHOR]

Soft X-ray intensity at 0.89 keV along the North Polar Spur (NPS) is shown to follow the extinction law due to the interstellar gas in the Aquila Rift by analysing the ROSAT archival data, which proves that the NPS is located behind the rift. The Aquila–Serpens molecular clouds, where the X-ray optical depth exceeds unity, are shown to have a mean LSR velocity of v = 7.33 ± 1.94 km s−1, corresponding to a kinematic distance of r = 0.642 ± 0.174 kpc. Assuming a shell structure, a lower limit of the distance to NPS is derived to be 1.01 ± 0.25 kpc, with the shell centre being located farther than 1.1 kpc. Based on the distance estimation, we argue that the NPS is a Galactic halo object. [ABSTRACT FROM AUTHOR]

The soft X-ray flux produced by solar axions in the Earth's magnetic field is evaluated in the context of the European Space Agency's XMM-Newton observatory. Recent calculations of the scattering of axion-conversion X-rays suggest that the sunward magnetosphere could be an observable source of 0.2-10 keV photons. For XMM-Newton, any conversion X-ray intensity will be seasonally modulated by virtue of the changing visibility of the sunward magnetic field region. A simple model of the geomagnetic field is combined with the ephemeris of XMM-Newton to predict the seasonal variation of the conversion X-ray intensity. This model is compared with stacked XMM-Newton blank sky datasets from which point sources have been systematically removed. Remarkably, a seasonally varying X-ray background signal is observed. The European Photon Imaging Camera count rates are in the ratio of their X-ray grasps, indicating a non-instrumental, external photon origin, with significances of 11σ (pn), 4σ (MOS1) and 5σ (MOS2). After examining the distribution of the constituent observations spatially, temporally and in terms of the accepted representation of the cosmic X-ray background, we conclude that this variable signal is consistent with the conversion of solar axions in the Earth's magnetic field, assuming the resultant photons are not strictly forward-directed, and enter the field of view of XMM-Newton. The spectrum is consistent with a solar axion spectrum dominated by bremsstrahlung- and Compton-like processes, distinct from a Primakoff spectrum, i.e. axion-electron coupling dominates over axion-photon coupling and the peak of the axion spectrum is below 1 keV. A value of 2.2 × 10-22 GeV-1 is derived for the product of the axion-photon and axion-electron coupling constants, for an axion mass in the µeV range. Comparisons, e.g., with limits derived from white dwarf cooling may not be applicable, as these refer to axions in the ~0.01 eV range. Preliminary results are given of a search for axion-conversion X-ray lines, in particular the predicted narrow features due to silicon, sulphur and iron in the solar core, and the 14.4 keV transition line from 57Fe. [ABSTRACT FROM AUTHOR]

Evolution in the X-ray luminosity–star formation rate (LX–SFR) relation could provide the first evidence of a top-heavy stellar initial mass function in the early Universe, as the abundance of high-mass stars and binary systems are both expected to increase with decreasing metallicity. The sky-averaged (global) 21-cm signal has the potential to test this prediction via constraints on the thermal history of the intergalactic medium, since X-rays can most easily escape galaxies and heat gas on large scales. A significant complication in the interpretation of upcoming 21-cm measurements is the unknown spectrum of accreting black holes (BHs) at high-z, which depends on the mass of accreting objects and poorly constrained processes such as how accretion disc photons are processed by the disc atmosphere and host galaxy interstellar medium. Using a novel approach to solving the cosmological radiative transfer equation (RTE), we show that reasonable changes in the characteristic BH mass affects the amplitude of the 21-cm signal's minimum at the ∼10–20 mK level – comparable to errors induced by commonly used approximations to the RTE – while modifications to the intrinsic disc spectrum due to Compton scattering (bound-free absorption) can shift the position of the minimum of the global signal by Δz ≈ 0.5 (Δz ≈ 2), and modify its amplitude by up to ≈10 mK (≈50 mK) for a given accretion history. Such deviations are larger than the uncertainties expected of current global 21-cm signal extraction algorithms, and could easily be confused with evolution in the LX–SFR relation. [ABSTRACT FROM PUBLISHER]

The cosmological 21 cm signal is a physics-rich probe of the early Universe, encoding information about both the ionization and the thermal history of the intergalactic medium (IGM). The latter is likely governed by X-rays from star formation processes inside very high redshift (z ≳ 15) galaxies. Due to the strong dependence of the mean free path on the photon energy, the X-ray spectral energy distribution (SED) can have a significant impact on the interferometric signal from the cosmic dawn. Recent Chandra observations of nearby, star-forming galaxies show that their SEDs are more complicated than is usually assumed in 21 cm studies. In particular, these galaxies have ubiquitous, sub-keV thermal emission from the hot interstellar medium (ISM), which generally dominates the soft X-ray luminosity (with energies ≲1 keV, sufficiently low to significantly interact with the IGM). Using illustrative soft and hard SEDs, we show that the IGM temperature fluctuations in the early Universe would be substantially increased if the X-ray spectra of the first galaxies were dominated by the hot ISM, compared with X-ray binaries with harder spectra. The associated large-scale power of the 21 cm signal would be higher by a factor of ∼3. More generally, we show that the peak in the redshift evolution of the large-scale (k ∼ 0.2 Mpc−1) 21 cm power is a robust probe of the soft-band SED of the first galaxies, and importantly, is not degenerate with their bolometric luminosities. On the other hand, the redshift of the peak constrains the X-ray luminosity and halo masses which host the first galaxies. [ABSTRACT FROM PUBLISHER]

The narrow Fe Kα line is one of the main signatures of the reprocessing of X-ray radiation from the material surrounding supermassive black holes, and it has been found to be omnipresent in the X-ray spectra of active galactic nuclei (AGN). In this work, we study the characteristics of the narrow Fe Kα line in different types of AGN. Using the results of a large Suzaku study, we find that Seyfert 2s have on average lower Fe Kα luminosities than Seyfert 1s for the same 10–50 keV continuum luminosity. Simulating dummy Seyfert 1s and Seyfert 2s populations using physical torus models of X-ray reflected emission, we find that this difference can be explained by means of different average inclination angles with respect to the torus, as predicted by the unified model. Alternative explanations include differences in the intensities of Compton humps, in the photon index distributions or in the average iron abundances. We show that the ratio between the flux of the broad and narrow Fe Kα line in the 6.35–6.45 keV range depends on the torus geometry considered, and is on average <25 per cent and <15 per cent for type-I and type-II AGN, respectively. We find evidence of absorption of the narrow Fe Kα line flux in Compton-thick AGN, which suggests that part of the reflecting material is obscured. We estimate that on average in obscured AGN the reflected radiation from neutral material is seen through a column density which is 1/4 of that absorbing the primary X-ray emission. This should be taken into account in synthesis models of the CXB and when studying the luminosity function of heavily obscured AGN. We detect the first evidence of the X-ray Baldwin effect in Seyfert 2s, with the same slope as that found for Seyfert 1s, which suggests that the mechanism responsible for the decrease of the equivalent width with the continuum luminosity is the same in the two classes of objects. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) interstellar medium (ISM) maps can be used to locate not only interstellar (IS) clouds, but also IS bubbles between the clouds that are blown by stellar winds and supernovae, and that are filled by hot gas. To demonstrate this and to derive a clearer picture of the local ISM, we compare our recent 3D maps of the IS dust distribution to the ROSAT diffuse X-ray background maps after removing heliospheric emission. In the Galactic plane, there is a good correspondence between the locations and extents of the mapped nearby cavities and the soft (0.25 keV) background emission distribution, showing that most of these nearby cavities contribute to this soft X-ray emission. Assuming a constant dust-to-gas ratio and homogeneous 106 K hot gas filling the cavities, we modeled the 0.25 keV surface brightness in a simple way along the Galactic plane as seen from the Sun, taking the absorption by the mapped clouds into account. The data-model comparison favors the existence of hot gas in the solar neighborhood, the so-called Local Bubble (LB). The inferred average mean pressure in the local cavities is found to be on the order of 10?000?cm-3?K, in agreement with previous studies, providing a validation test for the method. On the other hand, the model overestimates the emission from the huge cavities located in the third quadrant. Using CaII absorption data, we show that the dust-to-CaII ratio is very low in this region, implying there is a large quantity of lower temperature (non-X-ray emitting) ionized gas and, as a consequence, a reduction in the volume filled by hot gas, explaining at least part of the discrepancy. In the meridian plane, the main two brightness enhancements coincide well with the LB's most elongated parts and chimneys connecting the LB to the halo, but no particular nearby cavity is found towards the enhancement in the direction of the bright North Polar Spur (NPS) at high latitude. We searched in the 3D maps for the source regions of the higher energy (0.75 keV) enhancements in the fourth and first quadrants. Tunnels and cavities are found to coincide with the main bright areas; however, no tunnel or cavity is found to match the low-latitude b ? 8°, brightest part of the NPS. In addition, the comparison between the 3D maps and published spectral data do not favor the nearby cavities being located within ~200?pc as potential source regions for the NPS. Those examples illustrate the potential use of more detailed 3D distributions of the nearby ISM for interpreting the diffuse soft X-ray background. [ABSTRACT FROM AUTHOR]

In order to fully understand galaxy formation we need to know when in the cosmic history are supermassive black holes (SMBHs) growing more intensively, in what type of galaxies this growth is happening and what fraction of these sources are invisible at most wavelengths due to obscuration. Active Galactic Nuclei (AGN) population synthesis models that can explain the spectral shape and intensity of the cosmic X-ray background (CXRB) indicate that most of the SMBH growth occurs in moderate-luminosity (LX~ 1044 erg/s) sources (Seyfert-type AGN), at z~ 0.5−1 and in heavily obscured but Compton-thin, NH~ 1023cm−2, systems. However, this is not the complete history, as a large fraction of black hole growth does not emit significantly in X-rays either due to obscuration, intrinsic low luminosities or large distances. The integrated intensity at high energies indicates that a significant fraction of the total black hole growth, 22%, occurs in heavily-obscured systems that are not individually detected in even the deepest X-ray observations. We further investigate the AGN triggering mechanism as a function of bolometric luminosity, finding evidence for a strong connection between significant black hole growth events and major galaxy mergers from z~ 0 to z~ 3, while less spectacular but longer accretion episodes are most likely due to other (stochastic) processes. AGN activity triggered by major galaxies is responsible for ~60% of the total black hole growth. Finally, we constrain the total accreted mass density in supermassive black holes at z > 6, inferred via the upper limit derived from the integrated X-ray emission from a sample of photometrically selected galaxy candidates. We estimate an accreted mass density <1000 M⊙Mpc−3 at z~ 6, significantly lower than the previous predictions from some existing models of early black hole growth and earlier prior observations. [ABSTRACT FROM PUBLISHER]

X-ray surveys provide us with one of the least biased samples of Active Galactic Nuclei (AGNs) against obscuration. Here we present the most up-to-date AGN X-ray luminosity function (XLF) and absorption function over the redshift range from 0 to 5, using the largest, highly complete sample ever available obtained from surveys of various depth, depth, and energy bands. We utilize a maximum likelihood method to reproduce the count-rate versus redshift distribution for each survey, by taking into account the evolution of the absorbed fraction, contribution from Compton-thick AGNs, and AGN broad band X-ray spectra including reflection components from tori based on the luminosity and redshift dependent unified scheme. We find that the shape of the XLF at z ~ 1–3 is significantly different from that in the local universe, for which the luminosity dependent density evolution (LDDE) model gives the best description. These results establish the standard population synthesis model of the X-Ray Background (XRB), which well reproduces the source counts in both soft and hard bands, the observed fractions of Compton-thick AGNs, and the spectrum of the XRB. [ABSTRACT FROM PUBLISHER]

Context. Sources generating most of the X-ray background (XRB) are dispersed over a wide range of redshifts. Thus, statistical characteristics of the source distribution carry information on matter distribution on very large scales. Aims. We test the possibility of detecting the variation in the X-ray source number counts over the celestial sphere. Methods. A large number of Chandra pointings spread over both galactic hemispheres are investigated. We searched for all the point-like sources in the soft band of 0.5-2 keV and statistically assessed the population of sources below the detection threshold. A homogeneous sample of the number counts at fluxes above ∼10-15 erg s-1 cm-2 was constructed for more than 300 ACIS fields. The sources were counted within a circular area of 15 arcmin diameter. The count correlations between overlapping fields were used to assess the accuracy of the computational methods used in the analysis. Results. The average number of sources in the investigated sample amounts to 46 per field. It is shown that the source number counts vary between fields at a level exceeding the fluctuation amplitude expected for the random (Poissonian) distribution. The excess fluctuations are attributed to the cosmic variance generated by the large-scale structures. The rms variations of the source counts due to the cosmic variance within the 15 arcmin circle reach ∼8% of the average number counts. An amplitude of the potential correlations of the source counts on angular scales larger than the size of a single pointing remains below the noise level. [ABSTRACT FROM AUTHOR]

We present measurements of the Galactic halo’s X-ray emission for 110 XMM-Newton sight lines selected to minimize contamination from solar wind charge exchange emission. We detect emission from few million degree gas on ∼4/5 of our sight lines. The temperature is fairly uniform (median = 2.22 × 106 K, interquartile range = 0.63 × 106 K), while the emission measure and intrinsic 0.5–2.0 keV surface brightness vary by over an order of magnitude (∼(0.4–7) × 10−3 cm−6 pc and ∼(0.5–7) × 10−12 erg cm−2 s−1 deg−2, respectively, with median detections of 1.9×10−3 cm−6 pc and 1.5×10−12 erg cm−2 s−1 deg−2, respectively). The high-latitude sky contains a patchy distribution of few million degree gas. This gas exhibits a general increase in emission measure toward the inner Galaxy in the southern Galactic hemisphere. However, there is no tendency for our observed emission measures to decrease with increasing Galactic latitude, contrary to what is expected for a disk-like halomorphology. The measured temperatures, brightnesses, and spatial distributions of the gas can be used to place constraints on models for the dominant heating sources of the halo. We provide some discussion of such heating sources, but defer comparisons between the observations and detailed models to a later paper. [ABSTRACT FROM AUTHOR]

We report on the first 10 identifications of sources serendipitously detected by the Nuclear Spectroscopic Telescope Array (NuSTAR) to provide the first sensitive census of the cosmic X-ray background source population at ≳10 keV. We find that these NuSTAR-detected sources are ≈100 times fainter than those previously detected at ≳10 keV and have a broad range in redshift and luminosity (z = 0.020–2.923 and L10–40 keV ≈ 4 × 1041–5 × 1045 erg s−1); the median redshift and luminosity are z ≈ 0.7 and L10–40 keV ≈ 3 × 1044 erg s−1, respectively. We characterize these sources on the basis of broad-band ≈0.5–32 keV spectroscopy, optical spectroscopy, and broad-band ultravioletto- mid-infrared spectral energy distribution analyses. We find that the dominant source population is quasars with L10–40 keV > 1044 erg s−1, of which ≈50% are obscured with NH ≳ 1022 cm−2. However, none of the 10 NuSTAR sources are Compton thick (NH ≳ 1024 cm−2) and we place a 90% confidence upper limit on the fraction of Compton-thick quasars (L10–40 keV > 1044 erg s−1) selected at ≳10 keV of ≲33% over the redshift range z = 0.5–1.1. We jointly fitted the rest-frame ≈10–40 keV data for all of the non-beamed sources with L10–40 keV > 1043 erg s−1 to constrain the average strength of reflection; we find R < 1.4 for Γ = 1.8, broadly consistent with that found for local active galactic nuclei (AGNs) observed at ≳10 keV. We also constrain the host-galaxy masses and find a median stellar mass of ≈1011M⊙, a factor ≈5 times higher than the median stellar mass of nearby high-energy selected AGNs, which may be at least partially driven by the order of magnitude higher X-ray luminosities of the NuSTAR sources. Within the low source-statistic limitations of our study, our results suggest that the overall properties of the NuSTAR sources are broadly similar to those of nearby high-energy selected AGNs but scaled up in luminosity and mass. [ABSTRACT FROM AUTHOR]

We present new millimeter observations made with the IRAM interferometer and 30m telescope of the ionized wind from the massive young stellar object LkHα 101. Several recombination lines, including higher order transitions, were detected for the first time at radio wavelengths in this source. From three α-transitions, we derive an accurate value for the stellar velocity and for the first time, an unambiguous expansion velocity of the wind that is 55 kms-1. This velocity is much slower than reported previously, and the resulting mass loss rate is 1.8 × 10-6 M⊙ yr-1. The wideband continuum spectra and the interferometer visibilities show that the density of the wind falls off more steeply than what is compatible with constant-velocity expansion. We argue that these properties indicate that the wind is launched from a radially narrow region of the circumstellar disk, and we propose that slow speed and a steep density gradient are characteristic properties of the evolutionary phase, where young stars of intermediate and high mass clear away the gaseous component of their accretion disks. The recombination lines are emitted close to local thermal equilibrium, but the higher order transitions appear systematically broader and weaker than expected, probably because of impact broadening. Finally, we show that LkHα 101 shares many properties with MWC349, the only other stellar wind source where radio recombination lines have been detected, with some of them masing. We argue that LkHα 101 evades masing at millimeter wavelengths because of the disk's smaller size and unfavorable orientation. Some amplification may however be detectable at shorter wavelengths. [ABSTRACT FROM AUTHOR]

The Milky Way's million degree gaseous halo contains a considerable amount of mass that, depending on its structural properties, can be a significant mass component. In order to analyze the structure of the Galactic halo, we use XMM-Newton Reflection Grating Spectrometer archival data and measure Ovii Ka absorption-line strengths toward 26 active galactic nuclei, LMC X-3, and two Galactic sources (4U 1820-30 and X1735-444). We assume a β-model as the underlying gas density profile and find best-fit parameters of n엯 =0.46-0.35+0.74 cm-3, rc =0.35-0.27+0.29 kpc, and β = 0.71-0.14+0.13. These parameters result in halo masses ranging between M(18 kpc) = 7.5-4.6+22.0 × 108 M⊙ and M(200 kpc) = 3.8-0.5+6.0 × 1010 M⊙ assuming a gas metallicity of Z = 0.3Z⊙, which are consistent with current theoretical and observational work. The maximum baryon fraction from our halo model of fb = 0.07+0.03 -0.01 is significantly smaller than the universal value of fb = 0.171, implying the mass contained in the Galactic halo accounts for 10%-50% of the missing baryons in the Milky Way. We also discuss our model in the context of several Milky Way observables, including ram pressure stripping in dwarf spheroidal galaxies, the observed X-ray emission measure in the 0.5-2 keV band, the Milky Way's star formation rate, spatial and thermal properties of cooler gas (~105 K), and the observed Fermi bubbles toward the Galactic center. Although the metallicity of the halo gas is a large uncertainty in our analysis, we place a lower limit on the halo gas between the Sun and the Large Magellanic Cloud (LMC). We find that Z ≳ 0.2 Z⊙ based on the pulsar dispersion measure toward the LMC. [ABSTRACT FROM AUTHOR]

In order to understand the nature of the sources producing the recently uncovered cosmic infrared background (CIB) fluctuations, we study cross-correlations between the fluctuations in the source-subtracted CIB from Spitzer/IRAC data and the unresolved cosmic X-ray background from deep Chandra observations. Our study uses data from the EGS/AEGIS field, where both data sets cover an ≃8 ×45 region of the sky. Our measurement is the cross-power spectrum between the IR and X-ray data. The cross-power signal between the IRACmaps at 3.6 μm and 4.5 μ and the Chandra [0.5.2] keV data has been detected, at angular scales ≳20", with an overall significance of ≃3.8σ and ≃5.6σ, respectively. At the same time we find no evidence of significant cross-correlations at the harder Chandra bands. The cross-correlation signal is produced by individual IR sources with 3.6 μm and 4.5 μm magnitudes mAB ≳ 25.26 and [0.5.2] keV X-ray fluxes ⪡7 × 10-17 erg cm2 s-1. We determine that at least 15%-25% of the large scale power of the CIB fluctuations is correlated with the spatial power spectrum of the X-ray fluctuations. If this correlation is attributed to emission from accretion processes at both IR and X-ray wavelengths, this implies a much higher fraction of accreting black holes than among the known populations. We discuss the various possible origins for the cross-power signal and show that neither local foregrounds nor the known remaining normal galaxies and active galactic nuclei can reproduce the measurements. These observational results are an important new constraint on theoretical modeling of the near-IR CIB fluctuations. [ABSTRACT FROM AUTHOR]

Multiwavelength studies of radio relics at merger shocks set powerful constraints on the relics origin and formation mech-anism. However, for X-ray observations, a main difficulty is represented by the low X-ray surface brightness far out in the cluster outskirts, where relics are typically found. Here, we present XMM- Newton results from a 130-ks observation of CIZA J2242.8+5301, a cluster at z = 0.19 that hosts a double radio relic. We focus on the well-defined northern relic. There is a difference of ∼55% between the temperature we measure behind the relic, and the temperature measured with Suzaku. We analyse the reasons for this large discrepancy, and discuss the possibility of reliably measuring the temperature beyond the northern relic. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Apparently diffuse X-ray emission has been known to exist along the central quarter of the Galactic Plane since the beginning of X-ray astronomy; this is referred to as the Galactic Ridge X-ray emission (GRXE). Recent deep X-ray observations have shown that numerous X-ray point sources account for a large fraction of the GRXE in the hard band (2-8 keV). However, the nature of these sources is poorly understood. Using the deepest X-ray observations made in the Chandra bulge field, we present the result of a coherent photometric and spectroscopic analysis of individual X-ray point sources for the purpose of constraining their nature and deriving their fractional contributions to the hard-band continuum and Fe K line emission of the GRXE. Based on the X-ray color-color diagram, we divided the point sources into three groups: A (hard), B (soft and broad spectrum), and C (soft and peaked spectrum). The group A sources are further decomposed spectrally into thermal and non-thermal sources with different fractions in different flux ranges. From their X-ray properties, we speculate that the group A non-thermal sources are mostly active galactic nuclei and the thermal sources are mostly white dwarf (WD) binaries such as magnetic and non-magnetic cataclysmic variables (CVs), pre-CVs, and symbiotic stars, whereas the group B and C sources are X-ray active stars in flares and quiescence, respectively. In the logN-log S curve of the 2-8 keV band, the group A non-thermal sources are dominant above ≈10-14 erg cm-2 s-1, which is gradually taken over by Galactic sources in the fainter flux ranges. The Fe Ka emission is mostly from the group A thermal (WD binaries) and the group B (X-ray active stars) sources. [ABSTRACT FROM AUTHOR]

Emission from X-ray binaries (XRBs) is a major component of the total X-ray luminosity of normal galaxies, so X-ray studies of high-redshift galaxies allow us to probe the formation and evolution of XRBs on very long timescales (∼10 Gyr). In this paper, we present results from large-scale population synthesis models of binary populations in galaxies from z = 0 to ∼20. We use as input into our modeling the Millennium II Cosmological Simulation and the updated semi-analytic galaxy catalog by Guo et al. to self-consistently account for the star formation history (SFH) and metallicity evolution of each galaxy. We run a grid of 192 models, varying all the parameters known from previous studies to affect the evolution of XRBs. We use our models and observationally derived prescriptions for hot gas emission to create theoretical galaxy X-ray luminosity functions (XLFs) for several redshift bins. Models with low common envelope efficiencies, a 50% twins mass ratio distribution, a steeper initial mass function exponent, and high stellar wind mass-loss rates best match observational results from Tzanavaris & Georgantopoulos, though they significantly underproduce bright early-type and very bright (Lx > 1041) late-type galaxies. These discrepancies are likely caused by uncertainties in hot gas emission and SFHs, active galactic nucleus contamination, and a lack of dynamically formed low-mass XRBs. In our highest likelihood models, we find that hot gas emission dominates the emission for most bright galaxies. We also find that the evolution of the normal galaxy X-ray luminosity density out to z = 4 is driven largely by XRBs in galaxies with X-ray luminosities between 1040 and 1041 erg s-1. [ABSTRACT FROM AUTHOR]

ABSTRACT The diffuse appearance of the Galactic ridge X-ray emission has been puzzling since its discovery due to the lack of compelling theories for sustainable hot diffuse X-ray emission in the Galactic plane. Recently, Revnivtsev et al. claimed that ∼90 per cent of the 6.5-7.1 keV X-ray flux from a small section of a low-extinction region at 1°.4 south of the Galactic Centre has been resolved to discrete sources with [ABSTRACT FROM AUTHOR]

Aims. We study the spectral properties of the unresolved cosmic X-ray background (CXRB) in the 1.5-7.0 keV energy band with the aim of providing an observational constraint on the statistical properties of those sources that are too faint to be individually probed. Methods. We made use of the Swift X-ray observation of the Chandra deep field South complemented by the Chandra data. Exploiting the lowest instrument background (Swift) together with the deepest observation ever performed (Chandra) we measured the unresolved emission at the deepest level and with the best accuracy available today. Results. We find that the unresolved CXRB emission can be modeled by a single power law with a very hard photon index Γ = 0.1 ± 0.7 and a flux of 5+3.2 -2.6 × 10-12 erg s-1 cm-2 deg-2 in the 2.0-10 keV energy band (1σ error). Thanks to the low instrument background of the Swift-XRT, we significantly improved the accuracy with respect to previous measurements. Conclusions. These results point towards a novel ingredient in AGN population synthesis models, namely a positive evolution of the Compton-thick AGN population from local Universe to high redshift. [ABSTRACT FROM AUTHOR]

Context. Active galactic nuclei (AGN) are known to account for a major portion, if not all, of the cosmic X-ray background (CXB) radiation. The dominant sharp spectral feature in their spectra is the 6.4 keV fluorescent line of iron, which may contribute to as much as ~5−10% of the CXB spectral intensity at ~2−6 keV. Owing to cosmological redshift, the line photons detected at the energy E carry information about objects located at the redshift z = 6.4/E − 1. In particular, imprinted in their angular fluctuations is the information about the large-scale structure at redshift z. This opens the possibility of performing the Fe Kα line tomography of the cosmic large-scale structure. Aims. The goal of this paper is to investigate the feasibility of the Fe Kα line tomography of the large-scale structure. Methods. At any observed energy E, the 6.4 keV line photons are blended with continuum emission, which originates in objects located at many different redshifts and therefore contaminates and dilutes the tomographic signal. However, its contribution can be removed by doing observations at two nearby energy intervals and by calculating the power spectrum of the corresponding differential signal map. Results. We show that detection of the tomographic signal at ≳100σ confidence requires an all-sky survey by an instrument with an effective area of ~10 m2 and field of view of ~1deg2. The signal is strongest for objects located at the redshift z ~ 1 and at the angular scales corresponding to ℓ ~ 100−300, therefore an optimal detection can be achieved with an instrument having a rather modest angular resolution of ~0.1−0.5deg. For such an instrument, the CCD-type energy resolution of ~100−200 eV FWHM is entirely sufficient for the optimal separation of the signals coming from different redshifts. The gain in the signal strength that could potentially be achieved with energy resolution comparable to the line width is nullified by the photon counting and AGN discreteness noise. Among the currently planned and proposed missions, these requirements are best satisfied by LOFT, even though that it was proposed for an entirely different purpose. Among others, clear detection should be achieved by WFXT (~25−40σ) and ATHENA (~20−30σ). [ABSTRACT FROM AUTHOR]

This work measures the evolution of the iron content in galaxy clusters by a rigorous analysis of the data of 130 clusters at 0.1 < z < 1.3. This task is made difficult by a) the low signal-to-noise ratio of abundance measurements and the upper limits; b) possible selection effects; c) boundaries in the parameter space; d) non-Gaussian errors; e) the intrinsic variety of the objects studied; and f) abundance systematics. We introduce a Bayesian model to address all these issues at the same time, thus allowing cross-talk (covariance). On simulated data, the Bayesian fit recovers the input enrichment history, unlike in standard analysis. After accounting for a possible dependence on X-ray temperature, for metal abundance systematics, and for the intrinsic variety of studied objects, we found that the present-day metal content is not reached either at high or at low redshifts, but gradually over time: iron abundance increases by a factor 1.5 in the 7 Gyr sampled by the data. Therefore, feedback in metal abundance does not end at high redshift. Evolution is established with a moderate amount of evidence, 19 to 1 odds against faster or slower metal enrichment histories. We quantify, for the first time, the intrinsic spread in metal abundance, 18 ± 3%, after correcting for the effect of evolution, X-ray temperature, and metal abundance systematics. Finally, we also present an analytic approximation of the X-ray temperature and metal abundance likelihood functions, which are useful for other regression fitting involving these parameters. The data for the 130 clusters and code used for the stochastic computation are provided with the paper. [ABSTRACT FROM AUTHOR]

This paper investigates which constraints can be placed on the fraction of Compton-thick active galactic nuclei (AGN) in the Universe from modelling the spectrum of the diffuse X-ray background (XRB). We present a model for the synthesis of the XRB that uses as input a library of AGN X-ray spectra generated by Monte Carlo simulations. This is essential to account for the Compton scattering of X-ray photons in a dense medium and the impact of that process on the spectra of heavily obscured AGN. We identify a small number of input parameters to the XRB synthesis code that encapsulate the minimum level of uncertainty in reconstructing the XRB spectrum. These are the power-law index and high-energy cutoff of the intrinsic X-ray spectra of AGN, the level of the reflection component in AGN spectra, and the fraction of Compton-thick AGN in the Universe. We then map the volume of the space allowed to these parameters by current observational determinations of the XRB spectrum in the range 3−100 keV. One of the least-constrained parameters is the fraction of Compton-thick AGN. Statistically acceptable fits to the XRB spectrum at the 68% confidence level can be obtained for Compton-thick AGN fractions in the range 5−50%. This is because of degeneracies among input parameters to the XRB synthesis code and uncertainties in the modelling of AGN spectra (e.g. level of reflection fraction). The most promising route for constraining the fraction of Compton-thick AGN in the Universe is via the direct detection of those sources in high-energy (≳ 10 keV) surveys. We show that the observed fraction of Compton-thick sources identified in the Swift/BAT serendipitous survey limits the intrinsic fraction of Compton-thick AGN, at least at low redshift, to 10−20% (68% confidence level). We also make predictions on the number density of Compton-thick sources that current and future X-ray missions are expected to discover. Testing those predictions with data will place tight constraints on the intrinsic fraction of Compton-thick AGN as a function of redshift. [ABSTRACT FROM AUTHOR]

ABSTRACT We present the results of deep scans across the Galactic plane that were performed with RXTE/PCA in 2010-2011. We studied the stripe from bII = 0° to bII = −30° at lII ≈ 18.°5. We show that the variance of the cosmic X-ray background (CXB) at angular scales ∼1 deg2 is approximately 5 per cent, which is compatible with the variance expected from Poisson variations of a number of discrete sources within the considered field of view. We estimate the cumulative fractional emissivity of the Galactic ridge X-ray emission (GRXE) as (3.0 ± 0.3) × 1027 erg s−1 M [ABSTRACT FROM AUTHOR]

ABSTRACT At present, 30-40 per cent of the baryons in the local Universe is still undetected. According to theoretical predictions, this gas should reside in filaments filling the large-scale structure (LSS) in the form of a warm-hot intergalactic medium (WHIM), at temperatures 105-107 K, thus emitting in the soft X-ray energies via free-free interaction and line emission from heavy elements. In this work, we characterize the properties of the X-ray emission of the WHIM, and the LSS in general, focusing on the influence of different physical mechanisms, namely galactic winds (GWs), black hole feedback and star formation, and providing estimates of possible observational constraints. To this purpose, we use a set of cosmological hydrodynamical simulations that include a self-consistent treatment of star formation and chemical enrichment of the intergalactic medium, which allows us to follow the evolution of different metal species. We construct a set of simulated light cones to make predictions of the emission in the 0.3-10 keV energy range. We obtain that GWs increase the emission of both galaxy clusters and WHIM by a factor of 2. The amount of oxygen at average temperature and, consequently, the amount of expected bright O vii and O viii lines are increased by a factor of 3 due to GWs and by 20 per cent when assuming a top-heavy initial mass function. We compare our results with current observational constraints and find that the emission from faint groups and WHIM should account for half to all of the unresolved X-ray background in the 1-2 keV band. [ABSTRACT FROM AUTHOR]

Surveys above 10 keV represent one of the best resources to provide an unbiased census of the population of active galactic nuclei (AGNs). We present the results of 60 months of observation of the hard X-ray sky with Swift/Burst Alert Telescope (BAT). In this time frame, BAT-detected (in the 15-55 keV band) 720 sources in an all-sky survey of which 428 are associated with AGNs, most of which are nearby. Our sample has negligible incompleteness and statistics a factor of ~2 larger over similarly complete sets of AGNs. Our sample contains (at least) 15 bona fide Compton-thick AGNs and 3 likely candidates. Compton-thick AGNs represent ~5% of AGN samples detected above 15 keV. We use the BAT data set to refine the determination of the log N-log S of AGNs which is extremely important, now that NuSTAR prepares for launch, toward assessing the AGN contribution to the cosmic X-ray background. We show that the log N-log S of AGNs selected above 10 keV is now established to ~10% precision. We derive the luminosity function of Compton-thick AGNs and measure a space density of 7.9-2.9+4.1 x 10-5 Mpc-3 for objects with a de-absorbed luminosity larger than 2 x 1042 erg s-1. As the BAT AGNs are all mostly local, they allow us to investigate the spatial distribution of AGNs in the nearby universe regardless of absorption. We find concentrations of AGNs that coincide spatially with the largest congregations of matter in the local (≤85 Mpc) universe. There is some evidence that the fraction of Seyfert 2 objects is larger than average in the direction of these dense regions. [ABSTRACT FROM AUTHOR]

We present results of a study of the Galactic ridge X-ray emission (GRXE) in hard X-rays with the IBIS telescope onboard INTEGRAL in the region near the Galactic anticenter (GA) at l = 155°. We assumed a conservative 2σ upper limit on the flux from the GA in the 25-60 keV energy band of 1.25 × 10-10 erg s-1 cm-2 (12.8 mCrab) per IBIS field of view, or 6.6 × 10-12 erg s-1 cm-2 (0.7 mCrab) per degree longitude in the 135° < l < 175° region. This upper limit exceeds the expected GRXE intensity in the GA direction by an order of magnitude, given the near-infrared (NIR) surface brightness of the Milky Way in this region and the standard hard X-ray-to-NIR intensity ratio for the GRXE, assuming stellar origin. Based on the CGRO/EGRET surface brightness of the Galaxy above 100 MeV as a tracer of the cosmic-ray (CR) induced gamma-ray background, the expected GRXE flux in GA exceeds the measured 2σ upper limit by a factor of 4. Therefore, the non-detection of hard X-ray emission from the GA does not contradict the stellar nature of the GRXE, but is inconsistent with CR origin. [ABSTRACT FROM AUTHOR]

We present the analysis of 2.1 years of Fermi Large Area Telescope (LAT) data on 491 Seyfert galaxies detected by the Swift Burst Alert Telescope (BAT) survey. Only the two nearest objects, NGC 1068 and NGC 4945, which were identified in the Fermi first year catalog, are detected. Using Swift/BAT and radio 20 cm fluxes, we define a new radio-loudness parameter RX.BAT where radio-loud objects have log RX.BAT > -4.7. Based on this parameter, only radio-loud sources are detected by Fermi /LAT An upper limit to the flux of the undetected sources is derived to be ~2 x 10-11 photons cm-2 s-1 approximately seven times lower than the observed flux of NGC 1068. Assuming a median redshift of 0.031, this implies an upper limit to the γ-ray (1-100 GeV) luminosity of ≲3 x 1041 erg s-1. In addition, we identified 120 new Fermi/LAT sources near the Swift/BAT Seyfert galaxies with significant Fermi/LAT detections. A majority of these objects do not have Swift/BAT counterparts, but their possible optical counterparts include blazars, flat-spectrum radio quasars, and quasars. [ABSTRACT FROM AUTHOR]

Studying the average properties of active galactic nuclei (AGNs) most stellar populations is an important step in understanding the role of AGNs in galaxy evolution and the processes that trigger and fuel AGN activity. Here we calculate model spectral energy distributions that include emission from the AGN, the host galaxy stellar population, and dust enshrouded star formation. Using the framework of cosmic X-ray background population synthesis modeling, the model AGN hosts are constrained using optical (B band) and near-infrared (J band, 3.6 µm, 5.7 µm, 8.0 µm, and 24 µm) luminosity functions and number counts. It is found that at ɀ < 1, type 1 and type 2 AGN hosts have similar stellar populations, in agreement with the orientation-based unified model and indicative of secular evolution. At ɀ > l, type 2 AGN hosts are intrinsically different from type 1 AGN hosts, suggesting that the simple orientation-based unified model does not hold at ɀ > 1. Also, it is found that if Compton thick (CT) AGNs evolve like less obscured type 2 AGNs, then, on average, CT AGN hosts are similar to type 2 AGN hosts; however, if CT obscuration is connected to an evolutionary stage of black hole growth, then CT AGN hosts will also be in specific evolutionary stages. Multi-wavelength selection criteria of CT AGNs are discussed. [ABSTRACT FROM AUTHOR]

Finding and characterizing the population of active galactic nuclei (AGNs) that produces the X-ray background (XRB) is necessary to connect the history of accretion to observations of galaxy evolution at longer wavelengths. The year 2012 will see the deployment of the first hard X-ray imaging telescope which, through deep extragalactic surveys, will be able to measure the AGN population at the energies where the XRB peaks (~20-30 keV). Here, we present predictions of AGN number counts in three hard X-ray bandpasses: 6-10 keV, 10-30 keV, and 30-60 keV. Separate predictions are presented for the number counts of Compton thick AGNs, the most heavily obscured active galaxies. The number counts are calculated for five different models of the XRB that differ in the assumed hard X-ray luminosity function, the evolution of the Compton thick AGNs, and the underlying AGN spectral model. The majority of the hard X-ray number counts will be Compton thin AGNs, but there is a greater than tenfold increase in the Compton thick number counts from the 6-10 keV to the 10-30 keV band. The Compton thick population shows enough variation that a hard X-ray number counts measurement will constrain the models. The computed number counts are used to consider various survey strategies for the NuSTAR mission, assuming a total exposure time of 6.2 Ms. We find that multiple surveys will allow a measurement of Compton thick evolution. The predictions presented here should be useful for all future imaging hard X-ray missions. [ABSTRACT FROM AUTHOR]

We assess the possibility of detecting the warm-hot intergalactic medium in emission and characterizing its physical conditions and spatial distribution through spatially resolved X-ray spectroscopy, in the framework of the recently proposed DIOS, EDGE, Xenia, and ORIGIN missions, all of which are equipped with microcalorimeter-based detectors. For this purpose, we analyze a large set of mock emission spectra, extracted from a cosmological hydrodynamical simulation. These mock X-ray spectra are searched for emission features showing both the O VII Kα triplet and O VIII Lyα line, which constitute a typical signature of the warm-hot gas. Our analysis shows that 1 Ms long exposures and energy resolution of 2.5 eV will allow us to detect about 400 such features per deg2 with a significance ≥5σ and reveals that these emission systems are typically associated with density ∼100 above the mean. The temperature can be estimated from the line ratio with a precision of ∼20%. The combined effect of contamination from other lines, variation in the level of the continuum, and degradation of the energy resolution reduces these estimates. Yet, with an energy resolution of 7 eV and all these effects taken into account, one still expects about 160 detections per deg2. These line systems are sufficient for tracing the spatial distribution of the line-emitting gas, which constitute an additional information, independent from line statistics, to constrain the poorly known cosmic chemical enrichment history and the stellar feedback processes. [ABSTRACT FROM AUTHOR]

A signal from decaying dark matter (DM) can be unambiguously distinguished from spectral features of astrophysical or instrumental origin by studying its spatial distribution. We demonstrate this approach by examining the recent claim of Loewenstein and Kusenko regarding the possible DM origin of the 2.5 keV line in Chandra observations of the Milky Way satellite known as Willman 1. Our conservative strategy is to adopt, among reasonable mass estimates derived here and in the literature, a relatively large dark mass for Willman 1 and relatively small dark masses for the comparison objects. In light of the large uncertainty in the actual DM content of Willman 1, this strategy provides minimum exclusion limits on the DM origin of the reported signal. We analyse archival observations by XMM–Newton of M31 and Fornax dwarf spheroidal galaxy (dSph) and Chandra observations of Sculptor dSph. By performing a conservative analysis of X-ray spectra, we show the absence of a DM decay line with parameters consistent with those of Loewenstein and Kusenko. For M31, the observations of the regions between 10 and 20 kpc from the centre, where the uncertainties in the DM distribution are minimal, make a strong exclusion at the level above 10σ. The Fornax dSph provides a exclusion instead of a predicted 4σ detection, and the Sculptor dSph provides a 3σ exclusion instead of a predicted 2.5σ detection. The observations of the central region of M31 (1–3 kpc off-centre) are inconsistent with having a DM decay line at more than 20σ if one takes the most conservative among the best physically motivated models. The minimal estimate for the amount of DM in the central 40 kpc of M31 is provided by the model of Corbelli et al., assuming the stellar disc's mass to light ratio ∼8 and almost constant DM density within a core of 28 kpc. Even in this case one gets an exclusion at 5.7σ from central region of M31, whereas modelling all processed data from M31 and Fornax produces more than 14σ exclusion. Therefore, despite possible systematic uncertainties, we exclude the possibility that the spectral feature at ∼2.5 keV found in Loewenstein and Kusenko is a DM decay line. We conclude, however, that the search for DM decay line, although demanding prolonged (up to 1 Ms) observations of well-studied dSphs, M31 outskirts and other similar objects, is rather promising, as the nature of a possible signal can be checked. An (expected) non-observation of a DM decay signal in the planned observations of Willman 1 should not discourage further dedicated observations. [ABSTRACT FROM AUTHOR]

We investigate unresolved X-ray emission from M31 based on an extensive set of archival XMM–Newton and Chandra data. We show that extended emission, found previously in the bulge and thought to be associated with a large number of faint compact sources, extends to the disc of the galaxy with similar X-ray to K-band luminosity ratio. We also detect excess X-ray emission associated with the 10-kpc star-forming ring. The LX/SFR (star formation rate) ratio in the 0.5–2 keV band ranges from zero to , excluding the regions near the minor axis of the galaxy where it is times higher. The latter is likely associated with warm ionized gas of the galactic wind rather than with the star-forming ring itself. Based on this data, we constrain the nature of classical nova (CN) progenitors. We use the fact that hydrogen-rich material, required to trigger the explosion, accumulates on the white dwarf surface via accretion. Depending on the type of the system, the energy of accretion may be radiated at X-ray energies, thus contributing to the unresolved X-ray emission. Based on the CN rate in the bulge of M31 and its X-ray surface brightness, we show that no more than ∼10 per cent of CNe can be produced in magnetic cataclysmic variables, the upper limit being ∼3 per cent for parameters typical of CN progenitors. In dwarf novae, per cent of the material must be accreted during outbursts, when the emission spectrum is soft, and only a small fraction in quiescent periods, characterized by rather hard spectra. [ABSTRACT FROM AUTHOR]

We present the analysis of an observation by XMM–Newton that exhibits strongly variable, low-energy diffuse X-ray line emission. We reason that this emission is due to localized solar wind charge exchange (SWCX), originating from a passing cloud of plasma associated with a Coronal mass ejection (CME) interacting with neutrals in the Earth's exosphere. This case of SWCX exhibits a much richer emission-line spectrum in comparison with previous examples of geocoronal SWCX or in interplanetary space. We show that emission from O viii is very prominent in the SWCX spectrum. The observed flux from oxygen ions of is consistent with SWCX resulting from a passing CME. Highly ionized silicon is also observed in the spectrum, and the presence of highly charged iron is an additional spectral indicator that we are observing emission from a CME. We argue that this is the same event detected by the solar wind monitors Advanced Composition Explorer and Wind which measured an intense increase in the solar wind flux due to a CME that had been released from the Sun 2 d previous to the XMM–Newton observation. [ABSTRACT FROM AUTHOR]

Extensive measurements of the X-ray background (XRB) yield a reasonably reliable characterization of its basic properties. Having resolved most of the cosmic XRB into discrete sources, the levels and spectral shapes of its main components can be used to probe both the source populations and also alternative cosmological and large-scale structure models. Recent observations of clusters seem to provide evidence that clusters formed earlier and are more abundant than predicted in the standard Λ cold dark matter (ΛCDM) model. This motivates interest in alternative models that predict enhanced power on cluster scales. We calculate predicted levels and spectra of the superposed emission from groups and clusters of galaxies in ΛCDM and in two viable alternative non-Gaussian and early dark energy models. The predicted levels of the contribution of clusters to the XRB in the non-Gaussian models exceed the measured level at low energies and levels of the residual XRB in the 2–8 keV band; these particular models are essentially ruled out. Our work demonstrates the diagnostic value of the integrated X-ray emission from clusters, by considering also its dependences on different metallicities, gas and temperature profiles, Galactic absorption, merger scenarios and on a non-thermal pressure component. We also show that the XRB can be used for an upper limit for the concentration parameter value. [ABSTRACT FROM AUTHOR]

Active galactic nuclei (AGN) produce a dominant fraction of the soft X-ray background (SXB) at photon energies . If dust pervaded throughout the intergalactic medium, its scattering opacity would have produced diffuse X-ray haloes around AGN. Taking account of known galaxies and galaxy clusters, only a fraction of the SXB can be in the form of diffuse X-ray haloes around AGN. We therefore limit the intergalactic opacity to optical/infrared photons from large dust grains, with radii in the range , to a level to a redshift . Our results are only weakly dependent on the grain size distribution in this size range or the redshift evolution of the intergalactic dust. Stacking X-ray images of AGN can be used to improve our constraints and diminish the importance of dust as a source of systematic uncertainty for future supernova surveys which aim to improve the precision on measuring the redshift evolution of the dark energy equation-of-state. [ABSTRACT FROM AUTHOR]

With the goal of investigating the nature and the environment of the faint radio sources (at mJy level), here are presented results of X-ray identifications of Faint Imaging Radio Survey at Twenty centimetres (FIRST) in the 9 deg2 Boötes field of the National Optical Astronomy Observatory (NOAO) Deep Wide Field Survey (NDWFS), using data from the Chandra XBoötes survey. A total of 92 (10 per cent) FIRST radio sources are identified above the X-ray flux limit , and 79 optical counterparts are common to both the radio and X-ray sources. Spectroscopic identifications [obtained from the Sloan Digital Sky Survey (SDSS) survey] were available for 22 sources (27 per cent). The majority of these sources (59 per cent) are classified as broad line active galactic nuclei (BLAGNs), and 18 per cent as low-ionization nuclear emission-line regions (LINERs), 14 per cent as star-forming galaxies and one source classified as BL Lac object. Multiwavelength optical/infrared photometric data ( and ) were available for this field and were used to derive photometric redshift for the remaining 57 sources without spectroscopic information. Most of the radio–X-ray matches are optically extended objects in the R band with a red colour, their radio emission is associated with AGN activity hosted in massive early type host galaxies with a photometric redshift distribution peaking at . Based on the hardness ratio and X-ray luminosity, 37 sources (89 per cent) were classified as AGN-1, 19 as AGN-2, 12 as quasi-stellar object 1 (QSO-1), two as QSO-2 and nine sources as normal galaxies. While the majority of these sources have a hard X-ray luminosity , about one third of the sources have keV and therefore classified as QSO-1, 92 per cent of these objects are spectroscopically identified as QSOs. I found good agreement between the X-ray classification scheme and the classical AGN classification based on optical spectroscopic diagnostics. The majority (68 per cent) of the radio–X-ray matched population are found to have within region indicative of AGNs (Stocke et al.), 23 per cent with high X-ray to optical flux ratio , suggesting high redshift and/or dust obscured AGN, and 11 per cent of the radio–X-ray matches that are X-ray faint optically bright sources with , and most of these sources are optically extended. These objects are low z, normal galaxies or low-luminosity AGNs (LINERs). Five sources of the radio–X-ray matches are associated with the so called extremely red object (ERO) having a colour of . No clear correlation has been found between the colour and the hardness ratio. The very hard radio–X-ray matches (ignoring QSOs) all have low colour which could be a consequence of the correlation found between the hardness ratio and redshift, and the fact that low galaxies are mostly at low redshift. [ABSTRACT FROM AUTHOR]

We present a new method for determining the sensitivity of X-ray imaging observations, which correctly accounts for the observational biases that affect the probability of detecting a source of a given X-ray flux, without the need to perform a large number of time-consuming simulations. We use this new technique to estimate the X-ray source counts in different spectral bands (0.5–2, 0.5–10, 2–10 and 5–10 keV) by combining deep pencil-beam and shallow wide-area Chandra observations. The sample has a total of 6295 unique sources over an area of and is the largest used to date to determine the X-ray number counts. We determine, for the first time, the break flux in the 5–10 keV band, in the case of a double power-law source count distribution. We also find an upturn in the 0.5–2 keV counts at fluxes below about . We show that this can be explained by the emergence of normal star-forming galaxies which dominate the X-ray population at faint fluxes. The fraction of the diffuse X-ray background resolved into point sources at different spectral bands is also estimated. It is argued that a single population of Compton thick active galactic nuclei (AGN) cannot be responsible for the entire unresolved X-ray background in the energy range 2–10 keV. [ABSTRACT FROM AUTHOR]

We study the origin of unresolved X-ray emission from the bulge of M31 based on archival Chandra and XMM–Newton observations. We demonstrate that three different components are present. (i) Broad-band emission from a large number of faint sources – mainly accreting white dwarfs and active binaries, associated with the old stellar population, similar to the Galactic ridge X-ray emission of the Milky Way. The X-ray to K-band luminosity ratios are compatible with those for the Milky Way and for M32; in the 2–10 keV band, the ratio is . (ii) Soft emission from ionized gas with a temperature of about ∼300 eV and a mass of . The gas distribution is significantly extended along the minor axis of the galaxy, suggesting that it may be outflowing in the direction perpendicular to the galactic disc. The mass and energy supply from evolved stars and Type Ia supernovae is sufficient to sustain the outflow. We also detect a shadow cast on the gas emission by spiral arms and the 10-kpc star-forming ring, confirming significant extent of the gas in the ‘vertical’ direction. (iii) Hard extended emission from spiral arms, most likely associated with young stellar objects and young stars located in the star-forming regions. The (star formation rate) ratio equals , which is about ∼1/3 of the high-mass X-ray binary contribution, determined earlier from Chandra observations of other nearby galaxies. [ABSTRACT FROM AUTHOR]

We present calculations of the reflection of the cosmic X-ray background (CXB) by the Earth's atmosphere in the 1–1000 keV energy range. The calculations include Compton scattering and X-ray fluorescent emission and are based on a realistic chemical composition of the atmosphere. Such calculations are relevant for CXB studies using the Earth as an obscuring screen (as was recently done by INTEGRAL). The Earth's reflectivity is further compared with that of the Sun and the Moon – the two other objects in the Solar system subtending a large solid angle on the sky, as needed for CXB studies. [ABSTRACT FROM AUTHOR]